1. Technical Field
This invention relates to gas turbine engines and, more particularly, to variable area nozzles driven by shape memory alloy actuators therefor.
2. Background Art
Conventional gas turbine engines include a fan section and a core engine with the fan section having a larger outer diameter than that of the core engine. The fan section and the core engine are disposed sequentially about a longitudinal axis and are enclosed in a nacelle. An annular path of primary airflow passes through the fan section and the core engine to generate primary thrust. An annular path of duct or fan flow, disposed radially outward of the primary airflow path, passes through the fan section and exits through a fan nozzle to generate fan thrust.
The fan nozzles of conventional gas turbine engines have fixed geometry. The fixed geometry fan nozzles must be suitable for take-off and landing conditions as well as for cruise conditions. However, the requirements for take-off and landing conditions are different from requirements for the cruise condition. For cruise conditions, it is desirable to have a smaller diameter fan nozzle for increasing cruise performance and for maximizing fuel efficiency, whereas, for take-off and landing conditions, smaller diameter fan nozzles may cause an engine stall. Therefore, in many conventional engines, the cruise performance and fuel efficiency are often compromised to ensure safety of the gas turbine engine at take-off and landing.
Some gas turbine engines have implemented variable area nozzles. The variable area nozzles have the ability of having a smaller fan exit nozzle diameter during cruise conditions and a larger fan exit nozzle diameter during take-off and landing conditions. The existing variable area nozzles are either hydraulically or pneumatically actuated and result in complex mechanisms that require extensive maintenance. Most commercial aircraft prefer to avoid additional maintenance. Furthermore, the existing variable area nozzle mechanisms add significant weight to the engine. As is well known in the art, the extra weight adversely effects the overall performance of the aircraft. The additional weight reduces aircraft range and can result in additional fuel consumption for operation of the engine. Therefore, it is critical in gas turbine engine fabrication to avoid a weight increase, since the weight increase resulting from the addition of a variable area nozzle typically negates benefits gained from improved fuel efficiency resulting from the reduced diameter of the variable area nozzle during cruise conditions. Thus, although variable area nozzles have been introduced into some gas turbine engines, the use of existing variable area nozzles on most aircraft is not practical.
Therefore, it is desirable to develop a variable area nozzle that does not require extensive maintenance and does not add significant weight to the gas turbine engine.